You've probably heard of PrEP, maybe from a friend, an advertisement, or in health class. Regardless of how it came up, it was most likely referred to as something for men—specifically men who have sex with other men and want to avoid contracting HIV.

PrEP is a medication that can drastically lower someone’s risk of getting HIV. An estimated 120,000 people in the United States were taking it for that purpose at the start of 2017, according to a report from Gilead Sciences, PrEP’s manufacturer. But PrEP can help anyone at risk for HIV infection reduce that risk, no matter their gender or sexual orientation. Here's what you need to know about HIV, how PrEP protects against it, and whether you should consider taking this drug.

First, here’s a primer on HIV and how it’s transmitted, which is essential in understanding why PrEP is so effective.

HIV, or human immunodeficiency virus, is spread through certain bodily fluids (blood, semen, pre-seminal fluid, vaginal and rectal fluids, and breast milk) from a person who has HIV, according to the CDC. It attacks the immune system by killing off a person’s T cells, aka CD4 cells, which help the body battle infection. If you don’t have enough of these cells, you’re more vulnerable to illnesses.

Men who have sex with men are at the highest risk of contracting HIV because receptive anal sex has the highest potential transmission rate of all types of exposure except blood transfusions. According to the CDC, the risk of contracting HIV through unprotected receptive anal sex with a person who has it is 138 per 10,000 exposures. This is because the delicate lining of the rectum is so thin that it can allow HIV to enter the body more easily.

For comparison’s sake, the chance of getting HIV from a blood transfusion from a person who has the virus is 9,250 per 10,000 exposures. The odds drop to 63 per 10,000 exposures for needle sharing during intravenous drug use, 11 per 10,000 exposures for insertive anal sex, 8 per 10,000 exposures for receptive penile-vaginal intercourse, and 4 per 10,000 exposures for insertive penile-vaginal intercourse. The risk from receiving or giving oral sex is “so low that it is not possible to put a precise number on it,” and the risk from things like biting and sharing sex toys is “negligible.”

Certain factors, like using condoms when possible and not letting a person ejaculate inside of you, can lower your risk. Others, like having other sexually transmitted infections, may raise it.

Contrary to a popular and damaging myth, HIV is not a death sentence.

A person infected with HIV who gets treatment early can live a happy, active life for nearly as long as a person without it, according to the CDC. This is because HIV medications can do an incredible job of preventing the disease from progressing into AIDS.

A person is diagnosed with AIDS, or acquired immunodeficiency syndrome, when HIV has attacked their system to the point where their CD4 count drops below 200 per cubic millimeter of blood, according to the CDC. This allows life-threatening “opportunistic infections” like pneumonia, tuberculosis, and various cancers, to wreak havoc on a person’s health.

PrEP is the latest tool in the fight against HIV, and it works to prevent the infection from taking hold in the first place.

PrEP is a daily pill for HIV-negative people that, when taken consistently, can reduce the risk of HIV infection through sex by over 90 percent, according to the CDC. It can also lower the risk of HIV contraction in someone who injects drugs by over 70 percent when taken daily, according to the CDC.

PrEP comes as a pill under the brand name Truvada, which is actually a mix of the HIV medicines tenofovir and emtricitabine, and was first approved for use in the U.S. to prevent HIV in July 2012.

“These drugs were chosen because they have limited side effects, have few problems with drug resistance, and remain in the body for a relatively long time,” Mehri McKellar, M.D., an associate professor at Duke University’s Division of Infectious Diseases, tells SELF.

PrEP basically preloads your body with medicine, family doctor Adam Lake, M.D., an American Academy of HIV Medicine specialist working for Lancaster General Health in Lancaster, Pennsylvania, tells SELF. If exposure does occur, PrEP blocks pathways the HIV virus uses to infect your body. This prevents HIV from establishing itself so you can remain HIV negative.

It's worth noting that PrEP is different from post-exposure prophylaxis (PEP), which can be taken within 72 hours after possible HIV exposure (like a sexual assault) and continued for the prescribed 28 days. When taken as directed, this antiretroviral treatment can stop the infection from taking hold in someone’s system. PEP isn’t 100 percent effective in preventing HIV transmission, but the sooner it’s started, the better.

The problem is that some at-risk populations aren’t benefiting from PrEP as much as they could be.

Rates of new HIV diagnoses are highest for black and Hispanic/Latino gay and bisexual men, according to 2016 data from the CDC. Unfortunately, despite the fact that the majority of those living with HIV are people of color, 73 percent of people who filled PrEP prescriptions between January 2012 and September 2016 were white, according to Gilead Science’s 2017 report. “We need to make sure that African-Americans and Latinx are aware of PrEP and have access to this medication,” Dr. McKellar says.

And while the risk of HIV is highest among men who have sex with men, that doesn’t mean that other individuals (particularly those with other risk factors) can’t benefit from PrEP. That’s why Dr. Lake wishes more sex workers, intravenous drug users, and sexually active trans women knew about and had better access to PrEP, since their lifestyles put them at higher than average risk. So does being a cisgender woman or gender nonconforming person who’s had unprotected vaginal or anal sex with two or more partners in the last year, or who has a partner who has HIV or HIV risk factors.

If you’re concerned or meet certain risk factors, Dr. Lake suggests asking your doctor about PrEP. This doesn’t mean you definitely need to go on PrEP if you meet the qualifications. But it’s worth discussing with your doctor to see if your personal situation makes it a good choice for you, especially because you wouldn’t need to take it forever—only during periods of your life in which you’re at risk.

Of course, cost and side effects are both important considerations.

While PrEP’s cost varies depending on insurance, it can be prohibitively expensive, which is a hindrance to many people who need it. Without insurance, PrEP can cost up to $13,000 a year, according to the PrEP Facts, an awareness-raising project from the San Francisco AIDS Foundation and San Francisco Department of Health, among other groups.

There are, however, programs aimed at granting affordable access to those without insurance or those who need assistance paying their co-pays. If you have non-government insurance, you may be eligible for the Gilead Advancing Access program, which can offer up to $3,600 per year to help with co-pays based on your situation (you can learn more about the program’s terms here for the program’s full terms). If you don’t have insurance, are eligible for Medicaid, or your insurance denies your claim, the PrEP medication assistance program may help you get the drug for free. The CDC created a handy spreadsheet about paying for PrEP that you can check out, too. If all of this is confusing, you can ask your doctor if they have any information on how to lower the cost of PrEP or find an HIV-focused health center near you for more information.

Like any other medication, PrEP can have side effects. The most common ones are headache, abdominal pain, and weight loss, though in rarer cases it can cause kidney issues, liver and bone problems, and too much lactic acid buildup in your blood, which can be life-threatening. Make sure to discuss these risks thoroughly with your doctor before starting PrEP so you’re aware of any symptoms that are waving red flags.

Finally, the medication can only do its job well if you take it every day. It gets less effective if you don’t take it daily—commitment is key.

Even if PrEP isn’t right for you, it’s imperative that you know your HIV status, which is only possible through regular testing.

Regardless of your gender, orientation, or lifestyle, the CDC recommends that everyone between the ages of 13 and 64 get tested for HIV at least once. If you have certain risk factors like being sexually active, having been diagnosed with any sexually transmitted infections, and having had sex recently with someone whose history you don’t know, you should get tested at least once a year. The CDC also recommends that gay or bisexual men consider getting tested every three to six months.

There are various kinds of HIV tests available, including blood tests and oral swab tests. Some tests can give you results in 20 to 30 minutes, while other lab-based tests can take up to several days. Most forms of HIV testing can detect the virus in your body about 18 to 90 days after exposure, according to the CDC (so if you tested negative a few days after having unprotected sex, you should ask your doctor when to come in for another test). In general, tests that use blood from a vein can detect HIV sooner after infection than finger prick tests or oral swab tests.

The only way to know your and your partner’s HIV status is to get tested, and this is an especially necessary step to take if you’re considering not using condoms. Talking about HIV can feel scary, but let your partner know that you want to feel comfortable with them and getting tested will help quell your fears.

As a bonus, going in to get tested is the perfect time to ask your doctor whether you might be a good candidate for PrEP. If you think you’re at risk for contracting HIV—or are just curious about how well you’re protecting yourself, get tested and ask your doctor about what resources can help you have a healthier, safer sex life.